From a commercial standpoint, vinyl chloride homopolymers and copolymers of vinyl chloride and other monomers, such as vinyl acetate, are among the most important polymers presently produced.
In the suspension or emulsion process, the polymerization reaction is conventionally carried out in an agitated reactor, from which the slurry or latex produced as a result of the polymerization reaction is stripped in the reactor at a temperature below about 65.degree.C., or sometimes transferred to a hold tank, otherwise known as a stripper. A vacuum is applied to the reactor/stripper to remove monomer, and the slurry or latex is subsequently dewatered and dried to produce dried resin. The difficulty is, however, that this procedure, as conventionally practiced, results in the production of a slurry or latex of high vinyl chloride monomer content, for example, 100-15,000 parts per million (weight/weight), which results upon conventional processing in giving a dried (less than 1.0% moisture) resin with a VCM content of 100 ppm VCM or more.
An alternate and equally satisfactory approach is to treat the wet poly(vinyl chloride) in other than slurry or latex form; viz., "wet cake" form. Steam is used to treat the wet PVC homopolymer or copolymer either in process or after a drying step.
The removal of VCM from the wet PVC into equipment suitable for condensing, compressing or otherwise capturing the VCM reduces the concentration of VCM released into operating areas. The reduction of VCM levels in the air in working areas is required for safe plant operation. There is recent evidence that vinyl chloride monomer may be a cause of a rare liver cancer, known as angiosarcoma. Note Federal Register, Vol. 39, No. 92 -- Friday, May 10, 1974 -- pages 16896-16900.
An additional benefit is that the VCM captured is added to the recovered monomer pool, thereby improving polymerization efficiency and economy. The added captured monomer accounts for about an additional one to four percent of the total VCM charged.
Poly(vinyl chloride) homopolymer and copolymers of vinyl chloride and other monomers are conventionally subjected to hot operations, such as milling, calendering and extruding, which release vinyl chloride monomer associated with the homopolymer or copolymer. There is therefore a need in the art for a procedure whereby there can be produced a homopolymer or copolymer of reduced vinyl chloride monomer content. This invention answers that need.
In the suspension process, vinyl chloride homopolymer and copolymers of vinyl chloride and other monomers are prepared by suspending the vinyl chloride monomer alone or in a mixture with other monomers in water by suspending agents and agitation. The polymerization is started by means of a suitable free radical generating polymerization initiator, such as lauroyl peroxide, benzoyl peroxide, diisopropylperoxydicarbonate, tertiary butylperoxypivalate, azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, combinations of dialkylperoxydicarbonates and lauroyl peroxide, sulfonyl peroxides and the like. Suspending agents such as methylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, hydroxypropylmethylcellulose, hydrolyzed polyvinyl acetate, poly(vinyl alcohol), gelatin, methyl vinyl ether-maleic anhydride copolymers and combinations of the above are included in the reaction mixtures.
For the emulsion process, vinyl chloride homopolymer and copolymers of vinyl chloride and other monomers are prepared by conventional emulsion polymerization techniques. Free radical initiators, such as hydrogen peroxide, organic peroxides, persulfates, and redox systems are used. Surface active agents, such as alkyl sulfates, alkane sulfonates, alkylaryl sulfonates and fatty acid soaps, are employed to emulsify the vinyl chloride monomer and comonomers, if any. An alternate method of dispersion resin preparation is described in U.S. Pat. No. 2,981,722 to Enk et.al., granted Apr. 25, 1961.
When a copolymer of vinyl chloride and other monomers is prepared, the weight percent of the monomer, or mixtures of monomers, other than vinyl chloride can be up to thirty weight percent, based upon the total weight of the vinyl chloride and the other monomer or monomers copolymerized with the vinyl chloride. Among the monomers which can be copolymerized with the vinyl chloride are:
acrylic acid PA0 acrylonitrile PA0 n-butyl acrylate PA0 diallyl maleate PA0 dibutyl maleate PA0 diethyl fumarate PA0 dimethyl itaconate PA0 ethyl acrylate PA0 ethylene PA0 isobutylene PA0 maleic anhydride PA0 methacrylic acid PA0 methacrylonitrile PA0 methyl acrylate PA0 methyl vinyl ether PA0 2-ethylhexyl acrylate PA0 propylene PA0 triallyl cyanurate PA0 triallyl isocyanurate PA0 trimethylolpropane trimethacrylate PA0 vinyl acetate PA0 N-vinylcarbazole PA0 vinylidene chloride PA0 vinyl isobutyl ether PA0 N-vinylpyrrolidone
Further details concerning the production of vinyl chloride homopolymer and copolymers of vinyl chloride and other monomers by suspension and emulsion polymerization are set forth in Kirk-Othmer, Encyclopedia of Chemical Technology, Second Edition, Volume 21, pages 373-379.
In accordance with this invention, unreacted vinyl chloride is removed from wet PVC other than the slurry or latex, by the application of steam to the wet PVC, so that vinyl chloride vapor in a further amount is removed from the wet PVC. Treatment of the wet PVC with steam can be used in conjunction or in place of stripping of the PVC slurry or latex to complete VCM removal. When used in conjunction with stripping of the PVC slurry or latex, the steam treatment can be used to "polish" or remove the last traces of vinyl chloride monomer to no detectable levels as determined by gas chromatography.
It is pointed out that the processing of PVC and copolymers of vinyl chloride and other monomers with steam at 100.degree.C. or higher temperature is contrary to the teachings of the art, inasmuch as the art is aware that severe heating decomposes PVC and the copolymers.
An important feature of this invention is that the steam treatment of wet PVC can be completed within several minutes, so that polymerization turn-around time is not appreciably lengthened. The operation is preferably carried out at about atmospheric pressure using steam having a temperature of about 100.degree.C. to 125.degree.C.